Process for encapsulating microelectronic circuits with organic components

ABSTRACT

Special absorbers or getters are incorporated in hermetically sealed electronic circuits with organic components, for example, with parylene passivations, silver conductive adhesives, and sealing materials. The getter material, preferably BaAl 4 , is dispersed as an extremely fine-grained powder in a gas permeable, inert silicone rubber having a composition which varies according to the application. In short- or long-term thermal loading, for example in power hybrid systems, the proposed getters make it possible to intercept any corrosive fission products such as CO, CO 2 , NO/NO 2 , and water of reaction to avoid premature aging.

FIELD OF THE INVENTION

The invention relates to encapsulated microelectronic circuits orcircuit elements with organic components, whereby a getter isincorporated within the inner chamber for absorbing corrosive gases.

DESCRIPTION OF THE PRIOR ART

Due to the practical requirements imposed on such circuits or circuitelements, highly reliable electronic circuits are protected againstenvironmental influences, moisture, CO₂, SO₂, as well as other corrosiveagents by means of hermetic welding or soldering to a metal or ceramichousing. On the other hand, more and more organic materials are beingused for passivating, die-bonding, and for a covering against mechanicalshocks in the construction of electronic circuits. However, underthermal loading as occurs in hybrid power systems and partially, that isto a smaller extent, in small-signal circuits, a small gas generationmust be expected even in thermally stable epoxy-, polyurethane-, andsilicone-resins or elastomers of proven reliability in microelectronics.It is known through gas analysis, that the gases involved are mostly H₂O, CO₂, CO, NH₃, and organic acids, which lead to an electrolyticcorrosion of aluminum conduction paths and of bond wires, therebycausing failure of the circuit, in hybrid systems to which a voltage isapplied. The hereby resulting hydrogen as well as any remaining oxygenincrease the corrosion, so that these gases should be removed.

In low power hybrids, these gases partially precipitate as moisture andcause additional electrolysis when a voltage is applied. The formationof aging-, or fission-products may only be prevented to a limited extentin spite of exhaustive efforts in applying processing technology, forexample evacuation N₂ -flushing, or drying. The drying agents used inelectronics and known for example from the German Patent Publication(DE-OS) No. 3,112,564 or the molecular sieves based on zeolith, the useof which is known, for example from European Patent Publication (EP-Al)No. 0,113,282, only absorb the moisture and again release the moisturewhen the circuit is heated, that is under critical switching oroperating conditions.

Furthermore, it is suggested, for example in European Patent Publication(EP-A2) No. 0,025,647, to only use partially cross-linked siliconerubber as a getter or as a collector for dust particles. A dustcollector obviously does not work as a gas getter and does not absorbmoisture.

More suitable are highly active getter materials, which are distributedby vaporization to provide large surface areas, and which irreversiblyintercept any reactive gases, perhaps maintain the vacuum stabile andprevent any gas reaction.

Such highly active getters were developed for electron beam tubes forabsorbing corrosive gases and moisture traces. Preferably used for thispurpose are alkaline earth-, and zirconium metal-, and zirconiumhydride-rods, which are vaporized and which, when heated, react more orless strongly with the resulting fission gases, corresponding to barium,in the following reactions.

    ______________________________________                                        2 Ba + H.sub.2 O = Ba H.sub.2 + Ba O                                          5 Ba + 2 CO.sub.2 =                                                                              Ba C.sub.2 + 4 Ba O                                        3 Ba + CO =        Ba C.sub.2 + 2 Ba O                                        2 Ba + 2 NH.sub.3 =                                                                              Ba H.sub.2 + Ba (NH.sub.2).sub.2                           2 Ba + 2 R--COOH = Ba (R--COO).sub.2 + Ba H.sub.2                             Ba + H.sub.2 =     Ba H.sub.2                                                 ______________________________________                                    

Extremely fine grained BaAl₄ getters appear, at first, to be especiallysuited for use as the organic materials or the resulting fission gasesused in electronic circuits, because such getters are inert relative to"kovar" or chrome-nickel steel and are relatively stable with respect tonitrogen, but absorb or chemically absorb all remaining gases. However,a vaporization of the getter in a small housing is difficult.

OBJECT OF THE INVENTION

It is the aim of the invention to provide processes and materialssuitable for carrying out these processes, which make it possible for agetter to reliably render harmless the corrosive gases and interferingmoisture even in small electronic circuits which are to be encapsulated.

SUMMARY OF THE INVENTION

This aim is achieved according to the invention in that the gettermaterial is dispersed as an extremely fine-grained powder in a gaspermeable inert silicone rubber.

Achieving the above named aim therefore entails dispersing highlyactive, yet non-critically processable getters in an extremelyfine-grained distribution in a gas permeable, inert (with respect to thegetter, the gas, and the circuit), high purity carrier. Tests havedetermined that, on the one hand barium-aluminum alloys, and on theother hand low-viscosity, highly pure, thermally stable, slightlyadditively cross-linking two-component silicone gels satisfy theserequirements and may be mixed as desired. In a differentialthermoanalytic technique (DSC2), no reaction could be achieved if nomoisture was allowed to interact. A slow reaction determined by thediffusion velocity only starts when heat is applied, so that the getteris first activated during welding-in and during operation.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be clearly understood, it will now bedescribed, by way of example, with reference to the accompanyingdrawings, wherein:

FIGS. 1a to d show the structure of various forms of circuitsencapsulated by the process according to the invention;

FIG. 2 is a differential thermoanalytic diagram for a silicone gelwithout getter material;

FIG. 3 is a differential thermoanalytic diagram for a silicone gel withBaAl₄ in air;

FIG. 4 is a differential thermoanalytic diagram for a silicone gel withBaAl₄ in air with moisture;

FIG. 5 is a differential thermoanalytic diagram for a silicone gel withBaAl₄ in a humid environment.

DETAILED DESCRIPTION OF PREFERRED EXAMPLE EMBODIMENTS AND OF THE BESTMODE OF THE INVENTION

FIGS. 1a to 1d show the basic structure of a circuit encapsulated by theprocess according to the invention. In FIG. 1a, a microelectroniccomponent block B, which is, for example, passivated with Si₃ N₄,adheres to a substrate S, which, for instance, comprises Al₂ O₃. Thecomponent block B is electrically connected to the conductor paths LB ina known manner by bond wires D, which are made of, for example, gold orother highly conductive metal. The component block B and the bond wiresare located within a soft synthetic layer G, which comprises a siliconegel with a BaAl₄ filling. The layer G is covered by a film F comprisinga metal layer. Finally, the entire structure is encapsulated with epoxyresin H with an SiO₂ filling.

The soft synthetic layer G on the one hand serves as a mechanicalpadding of the component blocks and bond wires, and on the other hand italso serves as a carrier for the getter material. For this reason, itcomprises a material of the type initially described above and furtherspecified in the example embodiment.

It is to be understood that the process according to the invention mayalso be used for housings of a rigid material (FIGS. 1b to 1d).

FIGS. 1a to 1d show known structures having rigid housings. Thereby FIG.1b shows a widely known plastic or ceramic housing. The embodiments ofFIGS. 1c to 1d show metal or ceramic housings.

The two halves of the housing of FIG. 1b are, for example, connected byglass GL. The housings according to FIGS. 1c and 1d are welded by weldbeads SN or soldered by solder points L. It is common to all of thesestructures, that the existing empty inner space is at least partiallyfilled with the getter material according to the invention.

FIGS. 2 to 4 show differential thermoanalytic diagrams for a siliconegel as a carrier material and BaAl₄ as a getter material. All of thethermoanalyses were made with ΔT/Δt=constant, and equivalent testquantities and conditions were employed in each instance.

With regard to the interpretation of the diagrams it is to be noted thatthe vertical axis or ordinate depicts the heat evolution or change inmcal/sec. Temperatures in °K. are registered on the abscissa. A linearlyextending curve indicates that the heat flow is constant and no heat ofreaction or heat of transformation arises, dips indicate that heat isreleased in this region, for example through reaction of the getter withthe moisture.

FIG. 2 shows a differential thermoanalytic diagram for a silicone gelwithout a getter material. A heat evolution is not recognizable.

FIG. 3 shows a differential thermoanalytic diagram for a silicone gelwith BaAl₄ in air. A slight heat evolution is practically notrecognizable.

FIG. 4 shows a differential thermoanalytic diagram for a silicone gelwith BaAl₄ in air with moisture. The heat evolution in the range from380° K. to 440° K. is easily recognizable.

FIG. 5 shows a differential thermoanalytic diagram for a silicone gelwith BaAl₄ in a dampened or humid environment. The heat evolution in therange from 380° K. to 440° K. is sharply defined.

In the tests of FIGS. 4 and 5, a fresh mixture was used

In the following, three example embodiments will be described.

EXAMPLE 1

1 g Sil-gel-604 (producer: Wacker) with the components A and B presentin a ratio of 9:1 and 3 g barium-aluminum alloy having a medium grainsize of X≦40 μm are mixed under a dry nitrogen stream, degasified under10⁻² torr at 23°±3° for 5 minutes to form a getter paste which isimmediately used. The housing cover is brushed with a thin layer,approximately 5 to 10 mg/cm² of the getter paste. In approximately 1-2hours at room temperature or in approximately 5 minutes at 100°, thegetter is adhesively vulcanized.

EXAMPLE 2

The Sil-gel-604 components A and B are mixed in a ratio of 15:1.Otherwise the conditions are as in Example 1. Due to the hardnessdeficiency, the getter remains more gas permeable and stickier. Itbehaves as a dust getter or rather for intercepting interferingparticles.

EXAMPLE 3

Instead of Sil-gel-604, Sil-gel-600 and 601 are used. The Silgel-600 and601 are vulcanized in the heat so that the getter is already activated.It is to be understood that similar products of other origin can besubstituted, for example, the DOW CORNING types:

R-4-3117

XR-90-714 (721)

Q1-9205 (AI-9214)

Q3-6527 A and B

Instead of BaAl₄, zirconium metal and zirconium hydride are used as afine grained powder, whereby a more stable gas getter is obtained.

Although the invention has been described with reference to specificexample embodiments, it will be appreciated, that it is intended tocover all modifications and equivalents within the scope of the appendedclaims.

I claim:
 1. A microelectronic component, comprising a housing and anextremely fine grained powder mixture of zirconium metal and zirconiumhydride dispersed as a getter material in a gas permeable inert siliconerubber in said housing for binding corrosive gases.
 2. Themicroelectronic component of claim 1, wherein said extremely finegrained powder has a grain size within the range of 0.1 to 100 micron.3. The microelectronic component of claim 2, wherein said grain size iswithin the range of 5 to 50 micron.
 4. The microelectronic component ofclaim 1, wherein said silicone rubber is substantially ion-free havingand ion concentration of <50 ppm, which is weakly cross-linkable, andhaving a high gas permeation coefficient, said silicone rubber furtherbeing inert to said zirconium metal and zirconium hydride and to anycircuit elements in said housing.
 5. The microelectronic component ofclaim 4, wherein said silicone rubber is a silicone gel.